Pupil

Arithmetic expression evaluator written in Rust.

It implements a butchered Shunting-yard algorithm.

Pupil

To build the pupil executable, run cargo build. Append the --release switch as needed for optimized builds.

It has three different use cases:

• Interactive mode.

  Enter expressions and press enter to evaluate them.

  PATH/TO/CRATE/ROOT> cargo run
  Welcome to pupil, the arithmetic expression evaluator.
  
  Enter an expression, eg. 2 + 3, and press enter.
  Press ctrl-C to exit.
  
  >>> 2 + 3
  5
  >>> ^C
  

• Provide the expression to evaluate as command line arguments.

  This allows to evaluate a single expression and then exit.

  PATH/TO/CRATE/ROOT> cargo run -- 2 + 3
  Welcome to pupil, the arithmetic expression evaluator.
  Ok: 5
  

• Pipe input.

  Evaluates every line as separate expressions and prints the result line by line.

  PATH/TO/CRATE/ROOT> echo 2 + 3 | cargo run
  5
  

Library

This library can be found on crates.io. In your Cargo.toml put:

[dependencies]
pupil = "0.1"

A practical example can be found in src/pupil.rs.

Start things off by creating its environment which will hold the available builtins and the last answer.

extern crate pupil;

// Creates an empty environment.
let empty = pupil::Env::new();
// Creates an environment initialized with the default builtins.
let env = pupil::Env::default();

Create an expression and bind it to its environment.

let mut expr = pupil::Expr::new(&env);

// Feed it input, note that you cannot give it partial tokens.
expr.feed("2 +").unwrap();
expr.feed("3").unwrap();

// Calculate the final result.
let result = expr.result().unwrap();

You can perform the expression evaluation in a single step.

let result = pupil::Expr::new(&env).eval("2 + 3").unwrap();

// Note that you must update the ‘last answer’ manually like so:
env.ans = result;

That’s it.

License

MIT - See license.txt